I know we do. That would handle the more core things. But the Traits could then add some additionals. I'm certainly not meaning the core shouldn't be handled by a more generic method such as that.
But for example, a Protective Leader might plop down a Guard Tower in a new city immediately, even though that wouldn't be something other leaders would gain in their Free at Tech lists.
Basically, the new trait tag would just add some extra buildings to the Free at Tech mechanism based on what traits were owned by the player. In a best case scenario, it would add them rather sparingly from any given trait.